Bono says that Alicia Keys is "scary" at the premiere of her documentary.AP

Bono thinks that songstress Alicia Keys is “scary.”

“I was terrified the moment I met her. I was shaking in my boots,” the U2 frontman said at the premiere of Keys’ documentary, “Keep a Child Alive with Alicia Keys.” Bono added that Keys has a “lioness energy.”

Apparently, the two musicians met while recording a cover of Marvin Gaye’s “What’s Going On” with several other musicians. Bono was instantly taken with Keys.

“I was very moved by her singing of course,” he said. “But what was interesting was the hard questions afterwards, and I think it’s those hard questions that she asks that lead her.”

Keys’ documentary follows her trip to South Africa last year to visit sites funded by her HIV and AIDS charity, Keep a Child Alive. Keys brought five contest winners along with her to witness the effects of the AIDS pandemic in Africa. The documentary focuses on their experiences and the people they meet while there.

Keys explains, “Even if you never have been able to travel there, or if you never can, that doesn’t mean you can’t travel with us and really see it for yourself.”

Bono attended the premiere of the doc, and as Keys explained feeling highly empathic on the trip because she was pregnant at the time, Bono interjected, “She’s scary, isn’t she?”

He explained that he was only half joking.

“Everyone’s got heart, but actually you have to have the head for this,” he said. “You have to be tough and strategic. You have to be demanding. The money has to be spent well. All these things take a certain intellectual rigor … [Keys] can do anything.”